Car bombs are the most efficient way which terrorists have devised to kill or maim their victims. It is a method which is used frequently in Northern Ireland. Except on relatively rare occasions these devices have been attached to the vehicle by quite powerful permanent magnets. Several systems relying on the detection of the magnetic fields emitted by the magnets have been devised to detect these bombs. Clearly, these systems can only detect those devices which have been attached magnetically and are thus extremely limited in scope.